The exotic world of quantum matter:

Transcription

1 The exotic world of quantum matter: Spontaneous symmetry breaking and beyond. Talk at GK 1694 meeting Bad Liebenzell Peter Wölfle Institut für Theorie der Kondensierten Materie (TKM) Institut für Nanotechnologie (INT) Karlsruhe Institute of Technology

2 Quantum matter: a definition What is Quantum Matter? Solid or liquid matter ( Condensed Matter ) showing quantum properties on the macroscopic scale

3 Magnetic Quantum Matter Earliest example: ferromagnetic Iron, Nickel, Cobalt Magnetic field of a Permanent magnet Magnetic field of an electromagnet Permanent Ring Currents? Not understandable within classical physics! Spontane Magnetisierung Nickel T c =633 K Temperatur Phase transition Ferromagnet Paramagnet

4 Superconductivity of Metals Discovery of superconductivity by Heike Kamerlingh Onnes 1911 (Nobel prize 1913) Kelvin C High temperature -Superconductivity: Bednorz and Müller 1986 (Nobel prize 1987) Bi 2 Sr 2 Ca 2 Cu 3 O 10

5 Superfluidity of Quantum Liquids: Helium 4 Under normal pressure Helium remains a liquid down to absolute zero At temperatures T < 2.18 K Helium 4 becomes superfluid Pjotr Kapitza 1937 (Nobel prize 1978)

6 Superfluidity of Helium 3 Two stable Helium Isotopes, 4 He und 3 He (obtained through radioactive decay of Tritium) The Isotopes differ only by their nuclear spin: 4 He=(2p+2n), Spin=0 ; 3 He=(2p+1n), Spin=1/2 Although the 4 He and 3 He atoms have identical chemical properties (electron shells), the two liquids behave entirely differently at temperatures < 3 K! Superfluid Phases of 3 He appear at T < 2.6 mk: D. Lee, D. Osheroff, R. Richardson 1971 (Nobel prize 1996) Anisotropic, magnetic superfluid at temperatures of only 1/1000 of the transition temperature of 4 He Theory: A. J. Leggett (Nobel prize 2003)

7 Superfluidity of ultracold atomic gases Bose-Einstein-condensate (BEC) predicted 1925 discovered 1995 velocity distribution of atoms number of atoms in condensate Rb T c = 0,3 μk

8 Theory of Quantum Matter Theoretical framework used to describe Quantum matter 1. Non-relativistic Quantum Mechanics of the constituents of condensed matter: electrons, nuclei, atoms 2. Quantum Statistics of Many-Particle Systems: Fermions, Bosons, Anyons 3. Collective Behavior: spontaneous ordering, excitations

9 Quantum theory of electrons in solids Arnold Sommerfeld (1927) : Electrons in Metals modeled as system of identical quantum particles with negligible interaction (Fermigas) Ground state: Fermi sphere in momentum space (k<k F) ; Weak excitations near the Fermi surface (E k = ћ 2 k 2 /2m- E F << E F ) Felix Bloch (1928) ; Born,Oppenheimer (1927): Band structure of the energy spectrum of electrons in crystals Interaction with lattice vibrations Single particle theory of electrons in solids Explains many properties of normal metals qualitatively (modern quantitative formulation by Density Functional Theory (DFT)) Problem: Coulomb interaction between electrons unimportant?

10 Landau s Fermi liquid theory L.D. Landau (1957) Nobel prize 1962 free electron e - interaction adiabatic - qp quasi particle Effect of interaction absorbed by a handful parameters (effective mass, Landau parameters)

11 Concepts of Quantum Matter I: quasi-particles Description of the weakly excited states of a system of (strongly) interacting particles by mapping onto an effective model of nearly free quasi-particles Fermions (Spin ½) : Landau quasi-particles, Bogoliubov quasi-particles and/or Bosons (Spin 0,1) : phonons (sound or shear waves) plasmons (charge oscillations) excitons (bound particle-hole pairs) magnons (spin waves) orbitons (orbital waves)

12 Concepts of Quantum Matter II: spontaneous symmetry breaking Interacting electron systems subject to cooling may develop a long-range ordered state below a critical temperature T c The symmetry of the ordered state is lower than that of the disordered state: spontaneous symmetry breaking Example Ferromagnet: Orientation of magnetic moments cooling Emergence of preferred direction is breaking rotation symmetry in spin space

13 Concepts of quantum matter III: new quasiparticles as a consequence of spontaneous symmetry breaking Existence of order parameter field, e.g. local magnetization M(r,t) of a ferromagnet Elasticity of order parameter field allows for oscillations/wave excitations: - acoustical : Goldstone modes of dispersion ω = ck or similar (Spin waves; acoustical transverse phonons, Anderson-Bogoliubov mode,..) - optical : massive modes ω = const., k 0 (optical phonons, Cooper pair oscillations,..) new quasi particles: bosons Defects in order parameter field (Domain walls of the magnetization, vortices in a superconductor or superfluid,..) new topological excitations Gaps in the fermionic spectrum (Bogoliubov quasiparticles in a superconductor, qps in a metallic ferromagnet,..) new fermionic qps: particle number not conserved

14 Theory of superconductivity I BCS-Theory of electrons in a superconductor: Electrons are bound into Cooper pairs (Quasi-Bosons; extension >> particle distance!) and form a quantum coherent condensate. J. Bardeen, L. Cooper, R. Schrieffer (1957); Nobel prize 1971 Conventional superconductors: Orbital angular mom. L=0 Spin angular mom. S=0 Unconventional superconductors: Orbital angular mom. L 0 And/or Spin S 0 unique state (2L+1) x (2S+1) sub states order parameter matrix Energy gap in spectrum of fermionic excitations Ek Bogoliubov qp 2 2 k Superconductivity of Fermi systems is, similar to superfluidity of Bose systems a consequence of the quantum-mechanical entanglement of the Bosons (the Cooper pairs) in the condensate, encoded in the emergence of a macroscopic quantum phase ei Spontaneous breaking of U(1) gauge symmetry

15 Higgs mechanism in superconductors Excitations of the order parameter: Phase mode (Goldstone) gapless for neutral system (Anderson-Bogoliubov) gapped for charged system by coupling to longitudinal el.magn. field Amplitude mode (Higgs particle) gapful for neutral and charged system; threshold at no resonance vq plasma 2 Under special conditions Higgs particle well defined if Higgs 2 Higgs mechanism: transverse el.mag. field modes gapped (magnetic penetration depth ) 1/ PW Anderson, 1958

16 Higgs boson in the Raman spectrum of NbSe 2 Coupling of OP to optical phonons in Charge Density Wave material pulls Higgs boson energy down into energy gap NbSe 2 2 Pairbreaking continuum Pair vibration energy Higgs boson Th: P. B.Littlewood and C. M. Varma, 1982 Exp: R. Sooryakumar and M. V. Klein, 1980

17 p-wave pairing states in liquid Helium 3 Orbital angular momentum L=1 3x3 substates Spin triplet state (Pauli) S=1 SO(3) L x SO(3) S x U(1) A jμ order parameter Pseudo-isotropic state (Balian and Werthamer, 1963) Axial state (Anderson and Morel, 1961) A = R (n, ) jμ jμ i e spin-orbit rotation matrix R, broken symmetry with respect to relative spin-orbit rotations isotropic energy gap anisotropic dynamics l A jμ = 0 (n j+im j)dμ Preferred directions: m, n, l=mxn, in orbital space d perp. S, in spin space anisotropic energy gap Vollhardt, Wölfle, The superfluid phases of Helium 3

18 Collective modes and ultrasound in the B-phase Two squashing modes, each five-fold degenerate: quadrupolar oscillations of the isotropic gap squashing broadened by qp collisions Pair vibration energy Pairbreaking continuum Higgs mode squashing mode real-squashing mode 8/5 real-squ. Exp.: J. C. Wheatley et al., 1974 Th.: P. Wölfle, 1975 Higgs squashing Note supersymmetry relation: ( ) ( ) (2 ) (2 ) sq rsq fermion Nambu 1976 Exp.: Giannetta, 1980 Th.: Koch, Wölfle, 1980

19 Oscillations of the order parameter structure in the A-Phase of superfluid Helium 3 superfluid A-phase Wölfle (1973) A ( n im ) d, nˆ mˆ 0. j j j

20 Collective Modes and ultrasound in the A-phase Collective mode peaks in ultrasound absorption clapping normal-flapping

21 Majorana fermions at the core of vortices in He3-B Superfluid He3-B is a topologically non-trivial superfluid, supporting ring vortices of vorticity 1 (winding of the R-matrix) In the vortex core fermionic excitations may exist, which appear in time-reversal invariant pairs (Dirac) If the rings are linked, topology requires the existence of a zero mode Majorana fermions X-L Qi, T L. Hughes, S. Raghu, S-C Zhang, 2009

22 Standard model of Condensed Matter Theory Theory of Fermi or Bose liquid + spontaneous symmetry breaking = most successful concept of quantum matter

23 Beyond the Standard Model Quantum fluctuations in reduced dimensions may destroy Landau quasi particles Long range order Examples: Electrons in 1d (Quantum wire): Separation of Charge and Spin Landau quasi particle decays into Spinon und Holon Quantum Hall effect in 2d: Landau qp decays into fractional quasi particles Topological insulators Frustrated magnetic systems High temperature cuprate superconductors?

24 Quantum Hall effect Integer QHE: K. von Klitzing, M. Pepper, G. Dorda (1980) Fractional QHE: D.C. Tsui, H.L. Störmer, A.C. Gossard (1982) Theory: R.B. Laughlin (1983) Nobel prizes: 1985 K. von Klitzing 1998 R.B. Laughlin, H.L. Störmer, D.C. Tsui

25 Quantum Hall effect set-up Measurement of the electrical resistance in a magnetic field Longitudinal resistance: R xx =V x / I Hall resistance: R xy =V y / I

26 Energy spectrum of 2d electrons in magnetic field B Spectrum is: continuous / discrete ν : Filling factor of Landau levels L.D. Landau (1930)

27 Quantum Hall effect Plateaus in Hall resistance at multiples of Quantum resistance R Q h e 2 classical Integer QHE in 1 Rxy RQ, =1,2,.. Fractional QHE R fract xy q p 1 R q p p, pq 1 Q, q 2, 4,.. p 1, 2,.. V. Umansky und J. Smet (2000)

28 Integer Quantum Hall effect: edge states In the region of large electron density: Coulomb interaction between electrons negligible (screening) Disorder localizes electrons in the bulk of the QHE sample. Charge transport may occur only via edge channels, forming exactly one-dimensional, spin-polarized ideal quantum wires, of quantized conductance: 2 e GQ 1/ RQ h single edge channel: Dissipation free transport ν edge channels: in 1 Rxy RQ, =1,2,..

29 Fractional Quantum Hall effect: Composite Fermion = Electron + q flux quanta In case of small electron density: Coulomb interaction of electrons dominates q=2 : magnetic flux quantum h/e Composite Fermion absorbs part or all of the applied magnetic flux: Effective magnetic field: B B(1 2 ) eff Fermi liquid at Beff 0, 1 2 J. Jain (1989), Read, Halperin, Lee (1995)

30 QHE of Composite Fermions ( 1 ) 2 Coulomb integer QHE of CFs corresponds to fractional QHE of electrons p * p,, p integer pq 1 J. Jain (1989)

31 Transport properties of Composite Fermions Electrons in disordered potential of doping ions Composite fermions in a disordered magnetic field Focussing of CFs QH-sample with stripe pattern j x Smet et al. (1996) Weiss, von Klitzing, et al. (1989) Mirlin, Wölfle (1998)

32 Shot noise and fractional quasi particles: QHE Laughlin quasi particles: At ν=1/3 FQHE state have 3 flux quanta per electron Fractional quasi particle: vortex excitation carrying 1 flux quant 1/3 electron Detectable via shot noise measurement S=2e* I R effective fractional charge e*=e/3 Fractional statistics? Exp.: Glattli et al. (1997) de Picciotto et al. (1997)

33 Topological insulators May the integer Hall effect be realized even without external applied magnetic field? Yes! Following a proposal by C.J. Kane and E.J. Mele by way of energy splitting of spin states via spin-orbit-coupling (simulates the magnetic field). Dissipation free transport

34 Topological Insulators L. Molenkamp et al., 2007

35 Summary and Outlook Interacting quantum many-body systems (electrons, atoms,..) condense into ordered states featuring spontaneous symmetry breaking and supporting a zoo of new quasiparticles. The search for new types of order in new (artificially synthesized) materials with novel properties not encountered in nature goes on. More recently the search is focussing on exotic states of matter, characterized by more subtle types of order, sometimes with topological properties and/or with, fractional quasiparticles. The new concepts may be relevant for unraveling the puzzle of high temperature superconductivity (the holy grail!). Materials showing topologically protected quantum coherence properties are of interest in the context of quantum information processing. More is different P.W. Anderson